What are nitrating agents?

Hint: In organic chemistry, a general class of chemical processes called nitration. Nitration is the process of introducing a nitro group into an organic compound. The nitrating agents belong to this chemical process called nitration.

Complete answer: Nitration is a process which belongs to a general class of chemical process in organic chemistry. It is the process of introducing a nitro group into an organic compound. The term nitration is applied incorrectly to the different processes such as forming the nitrate esters among alcohols and nitric acid that takes place in synthesizing the nitroglycerin.
The nitration reaction serves to introduce one or more nitro groups \[( - N{O_2})\] into a reacting molecule.
The nitro group \[( - N{O_2})\] is used in the nitration reaction. A mixture of nitric acid \[(HN{O_3})\] and sulphuric acids \[({H_2}S{O_4})\] gives the most nitrating medium.
By cooling an amount of condensed sulphuric acid in an ice bath, the nitrate mixture is prepared. The concentration of nitric acid is applied steadily to an equivalent amount keeping the solution at a steady temperature.
Let’s see an example of a nitration reaction.
Nitration of benzene takes place in the presence of sulphuric acid \[({H_2}S{O_4})\] and nitric acid \[(HN{O_3})\] . This reaction would occur very slowly without sulphuric acids; it catalyzes (speed up) the reaction. Here, nitric acid \[(HN{O_3})\] acts as a nitrating agent. As the concentration of nitric acid \[(HN{O_3})\] and temperature of the reaction increases, we get multiple substituted nitrobenzenes.

Note:
Nitro group \[( - N{O_2})\] functions as a ring deactivator group. It becomes very helpful to have a nitrogen group present in a ring so it can be used as a directing group and as a masked amino group. It is also used in a replacement reaction.